This invention relates to tool holders and more specifically to an improved arrangement for mounting a tool holder in a spindle.
Tool holders are typically secured to spindles by positioning a conical shank on the tool holder in a conical opening or seat defined by the spindle. The tool holders also typically include a flange or a collar defining an annular seat which confronts an annular seat on the spindle surrounding the conical seat in the spindle. Since it is practically impossible to make interchangeable conical shanks that allow a fit both on the conical seat of the spindle and against the annular seat defined on the spindle in surrounding relation to the conical seat, it has been standard practice to specify a guaranteed clearance between the annular collar face and the annular spindle face so that the attachment stiffness between the spindle and the tool holder depends solely on the conical interconnection.
Although it is possible to machine the tool holder and spindle with high precision to achieve interface at the conical interconnection as well as at the flat annular seat for a given spindle and tool holder, it is virtually impossible to maintain the tolerances between the various interfaces to a degree such that it would be possible to interchange tool holders and spindles and yet retain positive interface at both the conical seat interface and the annular seat interface. In the absence of face contact, the stiffness depends totally on the tapered or conical interface. When the tapered seat in the spindle begins to wear out, effectively increasing the taper angle, the stiffness of the connection is drastically diminished. Stiffness is also drastically reduced when the mouth of the spindle hole expands during high rpm operation. Various attempts have been made to provide compliance between the conical shank and the conical seat of the spindle so as to allow the tool holder to seat in the spindle with positive interface at the conical shank as well as at the annular seat. These prior art compliant designs have several disadvantages. Specifically, the compliant designs are much more complex and thus much more expensive; since the taper and the holding portion are connected with a fit and are not solid, accuracy is reduced; and each clamping event involves some relative motions between the parts, thus allowing wear gradually to develop leading to further deterioration in accuracy.